Digital wireless transmission is sometimes used to transmit analog audio frequency signals and analog video signals. In digital wireless transmission, a digital signal train produced by digitizing an audio frequency signal or video signal is error-correction encoded to thereby develop a digital encoded train in a transmitter. With this digital encoded train, a carrier signal, which is a high-frequency signal, is modulated to provide a modulated signal. The modulated signal is transmitted through a transmitting antenna. At a receiver side, the modulated signal received by a receiving antenna is demodulated to provide the digital encoded train, from which the digital signal train is decoded. The decoded, digital signal train is converted back into the analog audio frequency signal or analog video signal in a digital-to-analog converter.
In wireless transmission where a transmitter and a receiver are used, a communication area in which proper wireless transmission is available is determined by transmission power of the transmitter and sensitivity of the receiver. Therefore, a receiver in such communication area cannot satisfactorily receive well a modulated signal from a transmitter outside the communication area. Because radio frequencies available for wireless microphones, when used as such transmitter, are limited, the frequency to be used for the wireless microphone must be selected from such frequencies. When the wireless microphone is being operated at one frequency, there may be an apparatus operating at the same frequency or at a frequency which interferes with the frequency of the wireless microphone. In such a case, a receiver for that wireless microphone cannot properly receive the modulated signal from the wireless microphone.
If the transmitter transmits the modulated signal from outside the communication area, or if the receiver is receiving the modulated signal transmitted from the transmitter within the communication area, but it is disturbed by other apparatus, the decoded output from the receiver may contain interruption in sound or picture or contain noise. However, it is not possible to know on the spot whether such interruption in sound or picture or noise is due to transmission from outside the communication area or due to disturbance by other apparatuses. In order to determine it, it is necessary to use a measuring device, e.g. a field strength measuring device, to monitor the reception field strength and know the signal receiving condition.
For example, a cellular phone is provided with a function to indicate when it is in an area outside the service area. Accordingly, if one cannot talk through the cellular phone, he or she can know it is because he or she is outside the service area, when he or she sees the screen on the cellular phone. Also, the cellular phone system employs a multi-channel access system, in which any one of a multiple frequencies can be used. Accordingly, if the reception by the cellular phone is disturbed by other apparatus, the frequency used by the cellular phone is automatically changed. Thus, it is less likely that cellular phones become to fail to communicate due to disturbance from other apparatus. In contrast, systems, such as a wireless microphone system, which neither change automatically the operating frequency nor have an out-of-communication-area indicating function, it is not possible to know the reception condition or to automatically improve the communication condition.
An object of the present invention is to provide a digital wireless receiver which can indicate the reception condition by taking advantage of properties of digital wireless transmission.